Internal combustion engine exhaust gas discharge system

ABSTRACT

Disclosed herein is an exhaust receiver for a two-cycle internal combustion engine. The exhaust receiver is generally in the form of a hollow U-shaped tube with each leg of the tube communicating with one of the exhaust ports of a two cylinder engine. Rarefaction waves are reflected to the exhaust ports from an expansion chamber at the web or bight of the U-tube during engine scavenging to reduce the pressures at the exhaust ports and facilitate removal of the exhaust gases. Compressive waves originating from one cylinder are reflected by the piston skirt of the other cylinder when the piston is the closing port. The reflected compression waves return to the originating cylinder just prior to the closing of the port of the originating cylinder during the compression stroke to increase the compression of the fuel-air mixtures.

United States Patent Boerma 1 Oct. 3, 1972 [72] lnventor: Michael J. Boerma,Waukegan,lll.

73 Assignee: Outboard Marine Waukegan, Ill.

22 Filed: April 9, 1970 21 Appl.No.: 26,843

Corporation,

[52] US. Cl. ..123/65 E, 60/313, 60/314 [51] Int. Cl. ..F02b 27/00, FOln 7/08 [58] Field of Search ..60/32 R, 32 M, 29, 31, 313,

60/314; 123/65 E, 59 B, 52 M [56] References Cited UNITED STATES PATENTS 1,698,468 l/l929 Beals ..123/59 B 1,889,964 12/1932 Johnson et a1. 123/59 B 2,643,510 6/1953 Kiekhaefer ..123/59 B 3,052,086 9/1962 Kiekhaefer et al. ..60/31 3,045,423 7/ 1962 Hulsebus ..60/32 R 3,453,824 7/1969 Biesecker ..60/32 M 2,476,816 7/1949 Carter ..l23/65 E 2,306,580 12/1942 Wilson ..123/65 E 3,385,052 5/1968 Holtermann et a1. .....60/32 R 3,434,280 3/1969 Burkhart ..60/32 R 3,507,301 4/1970 Larson ..60/32 M 3,470,690 10/ l 969 Thompson ..60/32 M FOREIGN PATENTS OR APPLICATIONS 886,556 10/1943 France ..60/32 M 678,119 3/1930 France ..60/32 M Primary Examiner-Wendell E. Burns Attorney-Robert E. Clemency, John W. Michael, Gerrit D. Foster, Bayard H. Michael, Paul R. Puemer, Joseph A. Gemignani and Andrew 0. Riteris [57] ABSTRACT Disclosed herein is an exhaust receiver for a two-cycle internal combustion engine. The exhaust receiver is generally in the form of a hollow U-shaped tube with each leg of the tube communicating with one of the exhaust ports of a two cylinder engine. Rarefaction waves are reflected to the exhaust ports from an expansion chamber at the web or bight of the U-tube during engine scavenging to reduce the pressures at the exhaust ports and facilitate removal of the exhaust gases. Compressive waves originating from one cylinder are reflected by the piston skirt of the other cylinder when the piston is the closing port. The reflected compression waves return to the originating cylinder just prior to the closing of the port of the originating cylinder during the compression stroke to increase the compression of the fuel-air mixtures.

18 Claims, 9 Drawing Figures PATENTEUBBI3 1912 3.695.238

SHEET 3 BF 4 INTERNAL COMBUSTION ENGINE EXHAUST GAS DISCHARGE SYSTEM BACKGROUND OF THE INVENTION The invention relates to tuned exhaust systems for internal combustion engines in which reflected waves are used to provide desirable pressures at the exhaust ports to increase engine efficiency. The invention is a further development of the subject matter disclosed in US. Pat. No. 3,385,052 which issued May 28, 1968 and pending application Ser. No. 733,l59 filed May 29, 1968, now US. Pat. No. 3,520,270.

Reference is hereby made to my earlier allowed application Ser. No. 746,079 filed July 19, 1968, now U.S. Pat. No. 3,543,509.

SUMMARY OF THE INVENTION The invention provides an exhaust arrangement for internal combustion engines which conveys reflected compressive waves to the engine exhaust ports during the compression stroke to increase the compression of the fuel-air mixture and conveys reflected rarefaction waves to the engine exhaust ports during the power stroke to facilitate engine scavenging and increase efficiency. The exhaust receiver of the invention is generally in the form of ahollow U-shaped tube with each leg of the tube communicating with the exhaust port of one of the cylinders. The exhaust receiver also includes an expansion chamber which is connected to and communicates with the juncture of the legs of the U-shaped tube. The expansion chamber also serves as the exhaust gas discharge passage.

When the exhaust port of a cylinder is uncovered during the power stroke, compressive waves are created at the exhaust port. During the time interval commencing when anexhaust port is opened by the piston on the power stroke thereby creating the compressive wave, and just prior to closing of the piston of the same exhaust port by the piston as the piston travels toward the cylinder head on the compression stroke, the compressive wave travels through the exhaust receiver, is reflected from the piston of a non-originating cylinder which is in registry with the exhaust port, and returns at the exhaust port of the originating cylinder just prior to closing the exhaust port by the piston. The length of the U-shaped tube is selected to provide the desired synchronization of the pulses and pistons.

The rarefaction waves are reflected from the expansion chamber and arrive at the exhaust ports during engine scavenging to lower the pressures and thereby facilitate removal of the exhaust gases from the cylinders.

The invention also includes an exhaust receiver with two exhaust passages separated by a hollow water cooled partition. The exhaust receiver includes an integral water overflow tube for discharge of circulating cooling water which surrounds the exhaust tube.

It is an object of the invention to provide an exhaust receiver for an internal combustion engine which provides low pressures at the engine exhaust ports during exhaust scavenging and high pressures at the exhaust ports during the compression stroke with the high pressures caused by compressive waves reflected from the piston skirt of an adjacent cylinder.

It is a further object of the invention to provide an exhaust receiver in the form of a hollow U-shaped tube with the legs of the tube communicating with the exhaust ports of the cylinders in a two-cylinder engine and in which the closed exhaust port of one cylinder reflects compressive waves through the U-shaped tube to the originating cylinder to increase pressures in the originating cylinder during the compression stroke.

Further objects and advantages of the present invention will become apparent from the following disclosure.

THE DRAWINGS FIG. 1 is a side elevational view of an engine which is shown schematically and which embodies the exhaust system of the invention.

FIG. 2 is a top view of the engine shown in FIG. 1.

FIG. 3 is a sectional view along line 33 of F IG. 1.

FIG. 4 is a side elevational view in fragmentary section of a marine propulsion device embodying the exhaust system-of the invention.

FIG. 5 is a fragmentary enlarged view of a portion of the lower unit shown in FIG. 4 and of the exhaust receiver.

FIG. 6 is a sectionalview along line 6-6 of FIG. 5.

FIG. 7 is a sectional view along line 77 of FIG. 5.

FIG. 8 is a sectional view along line 8-8 of FIG. 5.

FIG. 9 0s a sectional view taken along line 9-9 of FIG. 4.

DETAILED DESCRIPTION In the drawings, FIG. 1 discloses a conventional twocycle internal combustion engine which is generally designated 10 and which includes first and second cylinders, respectively 12 and 14 (FIG. 2), provided with first and second exhaust ports 16 and 18. The engine also includes a crank case fuel-air inlet port 20 and a transfer passage 22 for each cylinder. The engine also includes first and second pistons, respectively 24 and 26, which are reciprocally disposed in the cylinders 12 and 14 and connected by connecting rods 27 to a crank shaft 28 having crank pins30 and 32 which are angularly offset at Thus the pistons 24 and 26 conventionally operate in sequence as shown in FIG. 3 with piston 24 in the compression stroke while piston 26 is in the power stroke. The exhaust ports 16 and 18 are uncovered near the bottom of the power stroke and the ports 16 and 18 are closed immediately after initiation and during the first portion of the compression or power stroke.

In accordance with the invention, means are provided defining an exhaust receiver which communicates with the exhaust ports 16 and 18 and is adapted to convey compressive waves formed at one of the exhaust ports during the power stroke to the exhaust port of the other of the cylinders and convey reflected compressive waves from the exhaust port and piston of the other cylinder to the cylinder at which the compressive wave originated prior to closing of the exhaust port of the originating cylinder during the compression stroke of the originating cylinder. In the construction disclosed in FIGS. 1 and 2, the means comprises a generally U-shaped hollow tube 38 having legs 40 and 42. The leg 40 is connected to the cylinder 12 and is in communication with the exhaust port 16 and the leg 42 is connected to the cylinder 14 and is in communication with the exhaust port 18. The legs 40 and 42 of tube 38 have a predetermined length and a predetermined cross sectional area to provide the maximum tuning benefit at a predetermined engine rate of rotation and at a predetermined temperature of the exhaust receiver 38. Since the acoustic velocity of a wave depends on the cross sectional area of the exhaust gas passage and the temperature of the passage, these details must be considered as well as the length of the legs to obtain the desired pressures at the exhaust ports at a selected engine rate of rotation.

The means defining an exhaust receiver also includes an expansion chamber 44. The expansion chamber 44 communicates with the U-shaped tube 38 through a conduit 46 (FIG. 2) which is connected to the juncture 48 of the legs 40 and 42. The expansion chamber has an outlet 49 for underwater discharge or discharge to the atmosphere.

In operation of the embodiment shown in FIGS. 1, 2 and 3, as the piston 24 uncovers the exhaust port 16 on the power stroke, a sharp positive pulse compressive wave is created in the leg 42. The compressive wave travels through leg 42 and leg 40 and is reflected from the piston 26 which is in registry with exhaust port 18. The reflected compressive wave returns to the exhaust port just prior to closing of the exhaust port 16 by the piston 24 on the compression stroke. The presence of the compressive wave at the exhaust port 16 during closing of the port 16 by the piston increases the pressure in the cylinder, thereby increasing the compression of the fuel-air mixture prior to combustion to thus increase the efficiency and power output of the cylinder.

The exhaust arrangement also provides rarefaction waves at the exhaust ports during engine scavenging. When the piston 24 uncovers the exhaust port 16 during the power stroke, rarefaction waves are reflected from the expansion chamber 44 and returned to the exhaust port 16. The rarefaction waves reduce the pressure at the exhaust port 16 to facilitate removal of the exhaust gases from the cylinder. Compressive and rarefaction waves are similarly reflected to the exhaust ports 18 of cylinder 14.

FIGS. 4, 5, 6, 7 and 8 disclose a marine propulsion device 80 embodying the invention. The marine propulsion device includes an engine block 82 with first and second cylinders 84 and 86 respectively provided with first and second pistons 85 and 87 and first and second exhaust ports 88 and 90.

The marine propulsion device includes a lower unit 91 with a drive shaft housing 92 and gear case or lower, lower unit portion 94. In accordance with the invention, means are provided defining an exhaust system communicating with the first and second exhaust ports, the exhaust system being arranged to convey compressive waves originating or formed at the exhaust port 88 of the first cylinder 84 during the power stroke of the first cylinder 84 to the exhaust port 90 of the second cylinder 86 in timed sequence with the piston 87 in the second cylinder 86 so that the compressive wave arrives at the exhaust port 90 of the second cylinder 86 when the second cylinder is closed by the second piston 87 and so that the compressive wave is reflected from the second piston 87 and returns to the exhaust port 88 on the first cylinder 84 in timed sequence with the first piston and just prior to closing the exhaust port 88 of the first cylinder 84 during the compression stroke of the piston 85.

The means defining an exhaust system comprises first and second ports or exhaust outlets and 102 (FIGS. 5, 7 and 8) in the lower portionof the engine block 82. The ports 100 and 102 are located below the cylinders and inwardly offset from the exhaust ports 88 and 90. Exhaust flow from the exhaust port 88 to the port 100 is afforded by a passage 104 in the engine block which curves forwardly of the exhaust port 90 and which includes a recessed inner wall 106. To connect with or communicate with the engine block outlet 100, the wall- 106 has a wall portion 106A which extends inwardly and downwardly for a portion of its length toward the fore and aft centerline of the marine propulsion device. The passage 104 also includes spaced opposed wall portions 108 and 110 which extend outwardly from the wall 106 and which have generally the same contour. The passage 104 conveys the. exhaust gas flow from the port 88 around the exhaust port 90 to the engine outlet 100.

Exhaust flow from the engine port 90 to the engine block port or outlet 102 is provided by a passage 112 defined in part by an inwardly and downwardly inclined wall 114 (FIGS. 5 and 7) which extends toward the fore and aft centerline 107 (FIG. 6) at a sharper angle than wall portion 106A, and spaced opposed walls 110 and 1 18. The wall 1 l0 separates passages 104 and 112.

The passages 104 and 112 are enclosed by a cover (FIG. 5) which includes recessed wall portions 122 and 124 which have the general outline of wall portions 106 and 1 14, and are respectively located in spaced opposed relation with wall portions 104 and 124. As shown in FIG. 8, the wall 122 has an arcuate portion 126 which extends to wall 128 to direct exhaust flow into the engine block outlet 100. The wall 124 has an arcuate portion 129 which extends to adjacent engine block outlet 102. An intermediate wall 130 (FIG. 5) separates wall portions 124 and 122. The wall 130 is superimposed on wall portion 110 when the cover is assembled, thus maintaining the separation of the passages 104 and 112. The cover 120 also includes wall portions 131 and 133 (FIG. 7) which respectively project into the passages 104 and 112 adjacent the ports 88 and 90.

To direct the exhaust flow and pressure waves downwardly into passages 104 and 112 the cover 120 is provided with an arcuate wall portion 135 (FIG. 7) located opposite exhaust port 88 and an arcuate wall portion 136 located opposite port 90. To cool the cover 120, the cover 120 is enclosed by a water jacket (not shown).

To facilitate mounting of the drive shaft housing 92 to the engine block 82 and to convey the exhaust gases to a point vertically below the cylinders 84 and 86, the means defining an exhaust system also includes an adaptor or intermediate member 140 which is provided with wall means defining first and second intermediate passages 142 and 144. The passages 142 and 144 extend downwardly and inwardly toward the fore and aft center line of the drive shaft housing and propulsion unit as shown in FIGS. 7 and 8. The intermediate member 140 also includes a water passage 141 (FIG. 5)

which communicates with a water passage 143 in the block 82.

The exhaust system also includes an exhaust receiver or exhaust tube 150 (FIG. 4) having an intermediate wall or partition 152 which separates the receiver 150 into first and second passage portions 154 and 156. The exhaust receiver is arranged within the drive shaft housing 92 to provide a wall of cooling water between the exhaust receiver 150 and the drive shaft housing 92 to obtain the advantages disclosed in US. Pat. No. 3,198,162 which issued Aug. 3, 1965.

To cool the partition 152, the partition is provided with a passage or channel 153 which is upwardly open and in communication with water passage 141 in the intermediate member. One or more drainage apertures 155 (FIG. 5) at the lower end of the passage 153 afford circulation of water from the engine block 82 through water passage 141 and into the hollow interior 157 of the drive shaft housing.

The exhaust receiver 150 also includes a water overflow passage 157A for maintaining a wall of cooling water between the exhaust receiver 150 and the drive shaft housing 92 at a fixed level. The passage 157A is defined by a wall portion 159 which is located in adjacent relation to passage portion 156 and connected to the receiver 150. The upper end 161 of wall portion 159 has a lip 163 which forms a dam or spillway for the water surrounding the exhaust receiver 150. As shown in FIG. 9, the overflow passage 157A communicates with an aperture 165 in a horizontal internal wall portion 167 which is integral with the drive shaft housing 92. The internal wall portion 167 rigidifies the drive shaft housing and minimizes the transmission of vibrations from the exhaust gases to the drive shaft housing 92. The aperture 165 communicates with the gear case 94 through a passage 169 in the drive shaft housing 92. Discharge of water exteriorly of the gear case 94 is provided by apertures 171.

Above the water discharge of exhaust during idle operation is provided by a series of apertures 151 located in the exhaust receiver 150 adjacent the top of the exhaust receiver and communicating with exhaust passages 154 and 156. The exhaust gas percolates through water surrounding the overflow passage 157 and discharges through an aperture 152A into a receptacle 153A which has a discharge outlet 154A located above the water level of the water in the drive shaft housing and above the lip 163.

The passages 154 and 156 respectively register with the passages 142 and 144 in the intermediate member 140. The passages 154 and 156 extend downwardly and communicate at a juncture or pocket 156A (FIGS. 4 and 9) formed in part by upwardly concave wave wall means 158 which is connected to wall portion 167. The

pocket 156A is further defined by opposed spaced semi-circular side walls 166 which depend from an inturned wall portion 167 and are connected to the concave wall means 158. The pocket 156A provides an expansion chamber for the exhaust passages 154 and 156.

The exhaust receiver 150 is isolated from the drive shaft housing 92 by a resilient mount 167A which is supported on the horizontal wall portion 167. The concave wall means 158 is provided with a transverse slot 160 which affords discharge of the exhaust flow from both cylinders to an intermediate exhaust tube 162 which is resiliently supported by mounts 164 and 166 in the drive shaft housing. The intermediate exhaust tube 162 extends to a passage 168 in the gear case 94 with the passage 168 communicating with a hollow propeller hub 170 for underwater discharge of the exhaust gases.

In assembly, the exhaust receiver 150 is bolted to the intermediate member 140 and the intermediate member 140 is bolted to the engine block 82. The drive shaft housing 92 is then bolted to the intermediate member 140; The intermediate exhaust tube 162 is installed in the open lower end of the drive shaft housing 92 prior to bolting the gear case 94 to the drive shaft housing 92. r

The distance from the exhaust ports 88 through passage 104, passage 142 in the intermediate member 140, and through passage portion 154 to the upwardly concave compressive wave reflective wall means 158, from the wall means 158 to exhaust port through passage 156, passage 144 and passage 112, is selected to afford arrival of compressive waves formed at the exhaust ports of the cylinders during their power stroke at the non-originating cylinder when its exhaust port is closed by its piston and return of the wave to the originating cylinder in timed sequence with the piston in the originating cylinder so that the compressive wave returns just prior to closing of the exhaust ports by the piston in the originating cylinder on the compression stroke to increase compression of the fuel-air mixture. Rarefaction waves are created in the pocket 156A and arrive at the exhaust ports during scavenging of the cylinders to thus reduce the pressures in the exhaust system adjacent the ports and thereby facilitate removal of the exhaust gases.

' Various of the-features of the invention are set forth in the following claims.

What is claimed is:

. 1. A marine propulsion device comprising a twostroke engine block having first and second cylinders respectively having first and second exhaust ports respectively controlled by first and second pistons respectively and oppositely reciprocably movable in said first and second cylinders to open and close said ports such that when any one of the exhaust ports is open, the other exhaust port is closed, a drive shaft housing supporting said engine block, and exhaust receiver means for conveying a compressive wave emanating in response to opening of one exhaust port to .the other exhaust port for reflection thereby and, after reflection of the compressive wave by the piston closing the other exhaust port, for reconveying the reflected compressive wave back to said one exhaust port prior to closure thereof, said exhaust receiver means extending from said engine block and into said drive shaft housing and including first and second passages communicating respectively with said exhaust ports and at a juncture remote from said engine block and a concavewall portion spaced from said juncture for reflecting pressure waves traveling in said exhaust receiver.

2. A marine propulsion device in accordance with claim 1 wherein said means defining an exhaust receiver comprises wall means defining first and second exhaust outlets in said engine block, wall means in said engine block defining passage portions extending from adjacent said first and second exhaust ports to said first and second outlets in said engine block, said first and second exhaust receiver passages being in respective communication with said first and second outlets in said engine block, and said upwardly concave wall portion having a common exhaust discharge outlet for said first and second exhaust gas passages.

3. A marine propulsion device in accordance with claim 2 wherein said wall means defining said first and second exhaust gas passage portions in said engine block includes first and second recessed wall portions on said engine block extending angularly inwardly and downwardly for a portion of their length, spaced opposed contoured side wall portions along the edges of said recessed wall portions, and a cover having first and second recessed wall portions respectively located over said first and second recessed wall portions in said engine block.

4. A marine propulsion device in accordance with claim 2 including an intermediate member located between said engine block and the upper end of said drive shaft housing, said intermediate member including wall means defining first and second intermediate passages, said first and second intermediate passages being in communication with said first and second exhaust outlets in said engine block and in communication with said first and second exhaust receiver 1 passages.

5. A marine propulsion device in accordance with claim 4 wherein said first and second exhaust receiver passages are separated by a hollow upwardly open partition, said partition having a drainage aperture in communication with the interior of said drive shaft housing, and a cooling water passage in said intermediate member in communication with said upwardly open partition.

6. A device in accordance with claim 4 wherein said intermediate member has upper and lower surfaces and wherein said passages in said intermediate member extend downwardly and inwardly from said upper surface to said lower surface.

7. A marine propulsion device comprising a twostroke engine block having upper and lower cylinders with each cylinder having an exhaust port controlled by a piston, first and second passageways integrally formed in said engine block in respective communication with said cylinder exhaust ports, a drive shaft housing, an exhaust tube in said drive shaft housing, said exhaust tube having first and second vertically extending passages in communication with said first and second engine block passages and, at the lower end of said first and second exhaust tube passages, said exhaust tube further includes junction means for affording unrestricted exhaust gas flow between said first and second exhaust tube passages, said junction means in cluding a concave arcuate wall portion remote from said first and second passages and adapted to reflect pressure waves traveling through one of said exhaust tube passages into the other of said exhaust tube passages, and a slot in said arcuate wall portion forming a common exhaust outlet for said exhaust tube passages.

8. A marine propulsion device comprising an alternately operating two-stroke internal combustion engine having first and second cylinders with respective exhaust ports, and a lower unit having a drive shaft housing supporting said engine and a lower portion connected to said drive shaft housing, an exhaust receiver in said drive shaft housing including first and second exhaust passages respectively communicating with said exhaust ports, a partition separating said passages and including an upwardly open channel adapted to receive cooling water from said engine, a drainage aperture in said channel for discharging cooling water into the interior of said drive shaft housing, wall means on said exhaust receiver defining an overflow passage, an inturned wall on said drive shaft housing, an upwardly concave wall portion connected to said inturned wall and located below said exhaust receiver, a slot in said concave wall portion, an aperture in said inturned wall in communication with said overflow passage, and means for resiliently supporting said exhaust receiver on said inturned wall, an intermediate exhaust tube with upper and lower ends, said upper end communicating with said slot, and means for resiliently supporting the upper end of said intermediate exhaust tube on said inturned wall on said drive shaft housing and for resiliently supporting the lower end of said intermediate exhaust tube in said lower unit lower portion.

9. A two-stroke internal combustion engine comprising wall means defining a first cylinder having a first exhaust port and a second cylinder having a second exhaust port, first and second pistons respectively and oppositely reciprocably movable in said first and second cylinders to open and close said ports such that when any one of the exhaust ports is open, the other exhaust port is closed, and exhaust receiver means for conveying a compressive wave emanating in response to opening of one exhaust port to the other exhaust port for reflection thereby and, after reflection of the compressive wave by the piston closing the other exhaust port, for reconveying the reflected compressive wave back to said one exhaust port so as to arrive thereat just prior to closure thereof, said exhaust receiver means including first and second exhaust passage legs respectively communicating with said first and second exhaust ports, said exhaust receiver also including passage means communicating with said legs for directing unrestricted wave flow between said legs, said passage legs and said passage means having lengths affording said wave conveyance.

10. An engine in accordance with claim '9 wherein said passage legs and said passage means comprise a generally U-shaped tube.

11. An engine in accordance with claim 9 wherein said passage means also includes port means for generating a rarefaction wave from the compressive wave emanating from said one exhaust port and for reflecting the rarefaction wave to said one exhaust port prior to the time of arrival of the reflected compressive wave.

12. An internal combustion engine in accordance with claim 11 including an expansion chamber comm unicating with said port means so as to assist causing the generation and reflection of the rarefaction wave.

13. A marine propulsion device including a twostroke internal combustion engine comprising wall means defining a first cylinder having a first exhaust port and a second cylinder having a second exhaust port, first and second pistons respectively and oppositely reciprocably movable in said first and second cylinders to open and close said ports such that when any one of the exhaust ports is open, the other exhaust port is closed, a drive shaft housing supporting said engine, and exhaust receiver means for conveying a compressive wave emanating in response to opening of one exhaust port to the other exhaust port for reflection thereby and, after reflection of the compressive wave by the piston closing the other exhaust port, for reconveying the reflected compressive wave back to said one exhaust port prior to closure thereof, said exhaust receiver means comprising an exhaust receiver extending from said engine into said drive shaft housing and including first and second passages communicating respectively with said first and second exhaust ports and at a common juncture remote from said engine, and an upwardly concave wall portion located below said juncture.

14. A marine propulsion device in accordance with claim 13 wherein said lower unit includes an underwater exhaust discharge outlet and wherein said upwardly concave wall portion includes a slot, and a tube supported within said drive shaft housing and communicating between said slot and said discharge outlet.

15. A marine propulsion device comprising an alternately operating two-stroke internal combustion engine having first and second cylinders with respective exhaust ports, and a lower unit having a drive shaft housing supporting said engine and including an exhaust gas discharge outlet, an exhaust receiver in said drive shaft housing including first and second exhaust passages respectively communicating with said exhaust ports, an intumed wall on said drive shaft housing, means for supporting said exhaust receiver on said inturned wall, an upwardly concave wall portion connected to said intumed wall, located below said exhaust receiver, and communicating with said first and second exhaust passages, said concave wall portion having therein a downwardly open slot, an intermediate exhaust tube having an upper end communicating with said slot, and a lower end communicating with said exhaust gas discharge outlet, and means for supporting said intermediate exhaust tube within and from said drive shaft housing.

16. A marine propulsion device in accordance with claim 15 wherein said means supporting said exhaust receiver and said intermediate exhaust tube includes resilient members.

17. A method of operating a two-stroke internal combustion engine including a first cylinder having a first exhaust port, a second cylinder having a second exhaust port, and first and second pistons respectively and oppositely reciprocably movable in the first and second cylinders to open and close the ports such that when any one of the exhaust ports is open, the other exhaust port is closed, said method including the steps of conveying a compressive wave emanating in response to opening of one exhaust port to the other port and so that the compressive wave arrives at the other exhaust port when the other exhaust port is closed, reflecting the compressive wave by the piston closing the other exhaust port, and reconveying the reflected compressive wave back to the one exhaust port so as to arrive thereat just rior to closure thereof.

18. A me 0d In accordance with cla1m'17 including the further step of generating a rarefaction wave by the compression wave emanating from the one exhaust port and conveying the rarefaction wave to the one exhaust port prior to the time of arrival of the reflected compression wave. 

1. A marine propulsion device comprising a two-stroke engine block having first and second cylinders respectively having first and second exhaust ports respectively controlled by first and second pistons respectively and oppositely reciprocably movable in said first and second cylinders to open and close said ports such that when any one of the exhaust ports is open, the other exhaust port is closed, a drive shaft housing supporting said engine block, and exhaust receiver means for conveying a compressive wave emanating in response to opening of one exhaust port to the other exhaust port for reflection thereby and, after reflection of the compressive wave by the piston closing the other exhaust port, for reconveying the reflected compressive wave back to said one exhaust port prior to closure thereof, said exhaust receiver means extending from said engine block and into said drive shaft housing and including first and second passages communicating respectively with said exhaust ports and at a juncture remote from said engine block and a concave wall portion spaced from said juncture for reflecting pressure waves traveling in said exhaust receiver.
 2. A marine propulsion device in accordance with claim 1 wherein said means defining an exhaust receiver comprises wall means defining first and second exhaust outlets in said engine block, wall means in said engine block defining passage portions extending from adjacent said first and second exhaust ports to said first and second outlets in said engine block, said first and second exhaust receiver passages being in respective communication with said first and second outlets in said engine block, and said upwardly concave wall portion having a common exhaust discharge outlet for said first and second exhaust gas passages.
 3. A marine propulsion device in accordance with claim 2 wherein said wall means defining said first and second exhaust gas passage portions in said engine block includes first and second recessed wall portions on said engine block extending angularly inwardly and downwardly for a portion of their length, spaced opposed contoured side wall portions along the edges of said recessed wall portions, and a cover having first and second recessed wall portions respectively located over said first and second recessed wall portions in said engine block.
 4. A marine propulsion device in accordance with claim 2 including an intermediate member located between said engine block and the upper end of said drive shaft housing, said intermediate member including wall means defining first and second intermediate passages, said first and second intermediate passages being in communication with said first and second exhaust outlets in said engine block and in communication with said first and second exhaust receiver passages.
 5. A marine propulsion device in accordance with claim 4 wherein said first and second exhaust receiver passages are separated by a hollow upwardly open partition, said partition having a drainage aperture in communication with the interior of said drive shaft housing, and a cooling water passage in said intermediate member in communication with said upwardly open partition.
 6. A device in accordance with claim 4 wherein said intermediate member has upper and lower surfaces and wherein said passages in said intermediate member extend downwardly and inwardly from said upper surface to said lower surface.
 7. A marine propulsion device comprising a two-stroke engine block having upper and lower cylinders with each cylinder having an exhaust port controlled by a piston, first and second passageways integrally formed in said engine block in respective communication with said cylinder exhaust ports, a drive shaft housing, an exhaust tube in said drive shaft housing, said exhaust tube having first and second vertically extending passages in communication with said first and second engine block passages and, at the lower end of said first and second exhausT tube passages, said exhaust tube further includes junction means for affording unrestricted exhaust gas flow between said first and second exhaust tube passages, said junction means including a concave arcuate wall portion remote from said first and second passages and adapted to reflect pressure waves traveling through one of said exhaust tube passages into the other of said exhaust tube passages, and a slot in said arcuate wall portion forming a common exhaust outlet for said exhaust tube passages.
 8. A marine propulsion device comprising an alternately operating two-stroke internal combustion engine having first and second cylinders with respective exhaust ports, and a lower unit having a drive shaft housing supporting said engine and a lower portion connected to said drive shaft housing, an exhaust receiver in said drive shaft housing including first and second exhaust passages respectively communicating with said exhaust ports, a partition separating said passages and including an upwardly open channel adapted to receive cooling water from said engine, a drainage aperture in said channel for discharging cooling water into the interior of said drive shaft housing, wall means on said exhaust receiver defining an overflow passage, an inturned wall on said drive shaft housing, an upwardly concave wall portion connected to said inturned wall and located below said exhaust receiver, a slot in said concave wall portion, an aperture in said inturned wall in communication with said overflow passage, and means for resiliently supporting said exhaust receiver on said inturned wall, an intermediate exhaust tube with upper and lower ends, said upper end communicating with said slot, and means for resiliently supporting the upper end of said intermediate exhaust tube on said inturned wall on said drive shaft housing and for resiliently supporting the lower end of said intermediate exhaust tube in said lower unit lower portion.
 9. A two-stroke internal combustion engine comprising wall means defining a first cylinder having a first exhaust port and a second cylinder having a second exhaust port, first and second pistons respectively and oppositely reciprocably movable in said first and second cylinders to open and close said ports such that when any one of the exhaust ports is open, the other exhaust port is closed, and exhaust receiver means for conveying a compressive wave emanating in response to opening of one exhaust port to the other exhaust port for reflection thereby and, after reflection of the compressive wave by the piston closing the other exhaust port, for reconveying the reflected compressive wave back to said one exhaust port so as to arrive thereat just prior to closure thereof, said exhaust receiver means including first and second exhaust passage legs respectively communicating with said first and second exhaust ports, said exhaust receiver also including passage means communicating with said legs for directing unrestricted wave flow between said legs, said passage legs and said passage means having lengths affording said wave conveyance.
 10. An engine in accordance with claim 9 wherein said passage legs and said passage means comprise a generally U-shaped tube.
 11. An engine in accordance with claim 9 wherein said passage means also includes port means for generating a rarefaction wave from the compressive wave emanating from said one exhaust port and for reflecting the rarefaction wave to said one exhaust port prior to the time of arrival of the reflected compressive wave.
 12. An internal combustion engine in accordance with claim 11 including an expansion chamber communicating with said port means so as to assist causing the generation and reflection of the rarefaction wave.
 13. A marine propulsion device including a two-stroke internal combustion engine comprising wall means defining a first cylinder having a first exhaust port and a second cylinder having a second exhaust port, first and second pistons respectively and oppositely recIprocably movable in said first and second cylinders to open and close said ports such that when any one of the exhaust ports is open, the other exhaust port is closed, a drive shaft housing supporting said engine, and exhaust receiver means for conveying a compressive wave emanating in response to opening of one exhaust port to the other exhaust port for reflection thereby and, after reflection of the compressive wave by the piston closing the other exhaust port, for reconveying the reflected compressive wave back to said one exhaust port prior to closure thereof, said exhaust receiver means comprising an exhaust receiver extending from said engine into said drive shaft housing and including first and second passages communicating respectively with said first and second exhaust ports and at a common juncture remote from said engine, and an upwardly concave wall portion located below said juncture.
 14. A marine propulsion device in accordance with claim 13 wherein said lower unit includes an underwater exhaust discharge outlet and wherein said upwardly concave wall portion includes a slot, and a tube supported within said drive shaft housing and communicating between said slot and said discharge outlet.
 15. A marine propulsion device comprising an alternately operating two-stroke internal combustion engine having first and second cylinders with respective exhaust ports, and a lower unit having a drive shaft housing supporting said engine and including an exhaust gas discharge outlet, an exhaust receiver in said drive shaft housing including first and second exhaust passages respectively communicating with said exhaust ports, an inturned wall on said drive shaft housing, means for supporting said exhaust receiver on said inturned wall, an upwardly concave wall portion connected to said inturned wall, located below said exhaust receiver, and communicating with said first and second exhaust passages, said concave wall portion having therein a downwardly open slot, an intermediate exhaust tube having an upper end communicating with said slot, and a lower end communicating with said exhaust gas discharge outlet, and means for supporting said intermediate exhaust tube within and from said drive shaft housing.
 16. A marine propulsion device in accordance with claim 15 wherein said means supporting said exhaust receiver and said intermediate exhaust tube includes resilient members.
 17. A method of operating a two-stroke internal combustion engine including a first cylinder having a first exhaust port, a second cylinder having a second exhaust port, and first and second pistons respectively and oppositely reciprocably movable in the first and second cylinders to open and close the ports such that when any one of the exhaust ports is open, the other exhaust port is closed, said method including the steps of conveying a compressive wave emanating in response to opening of one exhaust port to the other port and so that the compressive wave arrives at the other exhaust port when the other exhaust port is closed, reflecting the compressive wave by the piston closing the other exhaust port, and reconveying the reflected compressive wave back to the one exhaust port so as to arrive thereat just prior to closure thereof.
 18. A method in accordance with claim 17 including the further step of generating a rarefaction wave by the compression wave emanating from the one exhaust port and conveying the rarefaction wave to the one exhaust port prior to the time of arrival of the reflected compression wave. 